Place the rinsed barley, sliced mushrooms, diced carrots, diced celery, diced onion, and minced garlic into the slow cooker.
Add the vegetable broth, then stir in the thyme, bay leaves, salt, and black pepper until everything is combined.
Cover the slow cooker and cook on low for 6 to 8 hours, until the barley is tender and most of the liquid has been absorbed.
Stir in the drained white beans during the final 30 minutes of cooking so they warm through without breaking down.
Remove the bay leaves, taste the cassoulet, and adjust salt or pepper if needed. Ladle into bowls and top with chopped parsley before serving.